Add tempfile and improve git error handling

Cargo.toml

@@ -30,6 +30,7 @@ toml = "^1.0"
 [dev-dependencies]
 assert_cmd = "^2.1.1"
 predicates = "^3.1.3"
+tempfile = "^3.10"
 
 [profile.release]
 opt-level = 3

src/git.rs

@@ -91,10 +91,20 @@ pub fn git_output(args: &[&str]) -> Result<String> {
         .with_context(|| format!("failed to run git {:?}", args))?;
 
     if !output.status.success() {
+        let stderr = String::from_utf8_lossy(&output.stderr);
+        let stderr = stderr.trim();
+        if stderr.is_empty() {
+            return Err(anyhow!(
+                "git {:?} exited with status {:?}",
+                args,
+                output.status.code()
+            ));
+        }
         return Err(anyhow!(
-            "git {:?} exited with status {:?}",
+            "git {:?} exited with status {:?}: {}",
             args,
-            output.status.code()
+            output.status.code(),
+            stderr
         ));
     }
 
@@ -137,8 +147,14 @@ pub fn staged_files() -> Result<Vec<String>> {
 }
 
 /// Get per-file staged diff.
+///
+/// `staged_files` returns paths relative to the repo root, but a bare pathspec
+/// is resolved by git relative to the current working directory. Anchor it
+/// with the `:/` top-level magic pathspec so this still works when commitbot
+/// is invoked from a subdirectory of the repo.
 pub fn staged_diff_for_file(path: &str) -> Result<String> {
-    let diff = git_output(&["diff", "--cached", "--", path])?;
+    let pathspec = format!(":/{path}");
+    let diff = git_output(&["diff", "--cached", "--", &pathspec])?;
     Ok(diff)
 }
 

tests/git.rs

@@ -1,7 +1,41 @@
 use commitbot::git::{
     find_first_pr_number, format_pr_commit_appendix_with_remote, parse_remote_repo,
-    short_commit_hash, split_diff_by_file, PrItem, PrSummaryMode,
+    short_commit_hash, split_diff_by_file, staged_diff_for_file, staged_files, PrItem,
+    PrSummaryMode,
 };
+use std::process::Command;
+
+/// Set up a throwaway git repo with a staged change in a nested file, and
+/// return its tempdir handle plus the nested directory's path.
+fn repo_with_staged_nested_file() -> (tempfile::TempDir, std::path::PathBuf) {
+    let dir = tempfile::tempdir().expect("tempdir");
+    let root = dir.path();
+
+    let run = |args: &[&str]| {
+        let status = Command::new("git")
+            .args(args)
+            .current_dir(root)
+            .status()
+            .expect("run git");
+        assert!(status.success(), "git {:?} failed", args);
+    };
+
+    run(&["init", "-q"]);
+    run(&["config", "user.email", "test@example.com"]);
+    run(&["config", "user.name", "Test"]);
+
+    let nested_dir = root.join("app").join("Models");
+    std::fs::create_dir_all(&nested_dir).expect("mkdir");
+    let file = nested_dir.join("OrderItem.php");
+    std::fs::write(&file, "original\n").expect("write");
+    run(&["add", "."]);
+    run(&["commit", "-q", "-m", "initial"]);
+
+    std::fs::write(&file, "original\nchanged\n").expect("rewrite");
+    run(&["add", "."]);
+
+    (dir, nested_dir)
+}
 
 #[test]
 fn parses_github_ssh_remote() {
@@ -136,6 +170,29 @@ fn short_commit_hash_short_input() {
     assert_eq!(result, "abc");
 }
 
+#[test]
+fn staged_diff_for_file_works_from_subdirectory() {
+    let (_dir, nested_dir) = repo_with_staged_nested_file();
+
+    let original_cwd = std::env::current_dir().expect("current dir");
+    std::env::set_current_dir(&nested_dir).expect("chdir into nested dir");
+
+    let result = (|| {
+        let files = staged_files()?;
+        assert_eq!(files, vec!["app/Models/OrderItem.php".to_string()]);
+
+        let diff = staged_diff_for_file(&files[0])?;
+        assert!(
+            diff.contains("+changed"),
+            "expected diff to contain the staged change, got: {diff:?}"
+        );
+        anyhow::Ok(())
+    })();
+
+    std::env::set_current_dir(original_cwd).expect("restore cwd");
+    result.expect("staged diff lookup from subdirectory");
+}
+
 #[test]
 fn pr_summary_mode_as_str() {
     assert_eq!(PrSummaryMode::ByCommits.as_str(), "commits");